Value of Ppk or Cpk when targeting thinner material

[Sean Kelley]

We have an interesting situation where a customer is asking us for a PPAP with capability studies that meet Ppk of 1.66. However their purchasing dept is asking us to target the lighteset gauge we can make and still guarantee meeting tolerences. Making this thinner material then really casues our Ppk to go down considerably. Anyone have any thoughts on this?

 

[Ron Rompen]

You're caught in the historical dilemna between cost and quality. Usually it's your OWN purchasing department that is driving for the lower cost material, but the same thought process applies.
Is the characteristic that requires capability designated as an SC? If not, then you at least have a negotiation point with your customer. If it is, then the same applies to Purchasing.
How stable and controlled is your process? Do you have any data that shows you are (or are not) capable? Get your history and data together, and then present your case to either your customer or their purchasing department. In the end, they will have to fight it out between them, and come up with a direction for you.

 

[Golfman25]

I would chart the cpk out at different gages. Then you can see where your process becomes "non-capable" and work with your customer from there. Good luck.

 

[Bev D]

Another example of the abomination that is the capability index.

Your customer's Purchasing and Supplier Quality groups are at odds. Time to get them together for a little chit-chat.

 

[bobdoering]

Some things to watch out for:

1. The further you attempt to move a distribution center from the tolerance center, you need to have less overall variation.

2. The less variation you have, the better measurement system you may need, so that the measurement system is not "eating up" tolerance within your distribution. The need for better resolution can ramp up quick in this scenario.

3. Recocgnize the factors in your Total Variance Equation, and consider any over-time variances that may not fall within your sample, but will hurt you in the future.

4. Can the supplier commit to tigher variation? If not, you have a smoke and mirrors situation brewing.